In the automatic dishwashing of dishes, glassware and flatware, the articles to be cleaned generally are first subjected to a main washing step, which is subsequently followed by one or more rising steps to remove adhering main-wash detergent ingredients and/or remaining soil. This rinse step drains any residual water from the surface of the articles and prevents the spotting, streaking, and filming of the washed and dried articles. It is customary to add in the rinsing step a rinse aid which causes the articles to dry more evenly and improves the visual appearance of the articles when dry.
When rinse water falls on a washed article, some residual water always is left on the article. The function of the rinse aid is to cause a sheeting action which drains the water remaining from the article. The sheeting action is induced because of hydrophobicity and the surface tension reduction properties of the rinse aid surfactant. In both household and institutional dishwashing machines, a good sheeting action provides the benefits of no spotting or streaking or filming on the washed article, reduces drying time, and removes any residual soil particulate.
Protein soil on the articles to be washed causes foaming in the dishwashing machine. Foam, or more specifically entrapped air, in the wash spray will reduce the mechanical efficiency of the spray function and interfere with maximum soil removal which in turn reduces the washing and cleaning ability of the dishwashing machine. Foam also adheres to the washed article and causes spotting and streaking if not removed. Soil can be trapped inside the foam and not get washed away. Thus, it is desirable for a rinse aid to have a food soil defoaming activity.
Because of the vigorous liquor movement and the spraying mode of washing and rinsing both in household and institutional dishwashing machines, a rinse aid formulation must be low-foaming. Rinse aids based on nonionic surfactants, for example, ethylene oxide and propylene oxide adducts of fatty alcohols, ethylene and propylene oxide block copolymers and other low foam nonionic surfactants are now widely used.
However, it has been found that the rinse aids containing nonionic surfactants may produce too much foam in the concentration ranges required for an adequate wetting effect. This leads to problems in the dishwashing machines through excessive foaming.
Most surfactants used in commercial rinse aids are hydrophobic in nature and thus, they require a hydrotrope to keep these surfactants in solution during storage. A hydrotrope is used to perform the necessary solubilization of any hydrophobic nonionic surfactants in water. Some commonly used hydrotropes are sodium naphthalene sulfonate, sodium xylene sulfonate or sodium cumene sulfonate.
A typical commercial batch of rinse aid is produced by mixing the nonionic rinse aid surfactants with the hydrotrope. A dispersing agent is then mixed in the batch. Finally, the required amount of water is added and mixed until a homogenous solution is obtained. Dyes, perfumes, sequestering agents (which inhibit precipitation of water hardness salts), and preservatives are optionally added to the batch. These rinse aids are injected into the final rinse water at a concentration of about 50 to 500 parts per million.
Generally a rinse aid or rinse aid composition is defined as including the surfactant and the solubilizing system for the surfactant. A rinse aid concentrate includes the rinse aid composition, a dispersing agent for the surfactants and water plus other optional ingredients as desired to accomplish specific purposes. A rinse concentrate is usually 40%-100% active ingredients. Commercial distributors and vendors purchase the concentrate, add water to dilute it to 8%-40% active ingredients and sell it. Usually a rinse solution is used to refer to the fully diluted aqueous solution which is sprayed on the articles to be washed within the machine spray washer. These terms will be used throughout the application.
Nonionic surfactants known to be low foaming include for example block copolymers of polyoxypropylene-polyoxyethylene or ethylene oxide and propylene oxide adducts of fatty alcohols such as poly(oxyalkylated) alcohols which are represented by the formula: EQU RO--(PO).sub.x --(EO).sub.y --(PO).sub.z --H (I)
wherein
R is a linear, alkyl hydrocarbon having an average of from about 7 to about 10 carbon atoms; PA0 PO stands for propylene oxide groups ##STR1## and EO stands for ethylene oxide groups (CH.sub.2 --CH.sub.2 --O); PA0 x is an integer having a value from 1 to 6; PA0 y is an integer having a value from 4 to 15; and PA0 z is an integer having a value from 4 to 25.
U.S. Pat. No. 4,443,270 discloses a rinse aid composition for use in automatic dishwashing machines comprising a low foaming ethoxylated nonionic surfactant, an organic chelating agent, a hydrotrope-water solubilizing system and magnesium, zinc, tin, bismuth or titanium ions in the form of a water soluble salt.
While low foam surfactants have improved the sheeting action of rinse water on surfaces, they have not completely eliminated spotting and streaking problems. It is desirable to make the surface of the article to be washed as hydrophobic as possible because a more hydrophobic surface will enhance the removal of the residual water and this reduces spotting, streaking and filming on the washed articles when they are dried.